“It involves ripping the soil to a depth of about 30 centimetres to put manure into the lower layers.”

Subsoil manuring is particularly effective in ‘duplex soils’ that have contrasting layers of sandy topsoil and clay subsoil.

The dense clay restricts the movement of air and water, and impedes root growth.

Around 30 per cent of agricultural soils in Tasmania are duplex soils, mainly in the Midlands and south-east Tasmania.

“Duplex soils can be difficult to manage because they get rock hard when they dry out above the clay layer in spring, and waterlogged on top of the clay in winter,” NRM North Regional Landcare Facilitator Adrian James said.

NRM North provided seed funding for Tasmania’s first broadacre field trial of subsoil manuring at Epping Forest in 2015, in a joint research project with the institute.

“The four different treatments we trialled were 15 tonnes per hectare of poultry manure, seven tonnes per hectare of poultry manure and straw, a control treatment of deep ripping with no fertiliser, and a standard treatment of no ripping with no fertiliser,” Mr James said.

Additional funding from the Grains Research & Development Corporation enabled a trial crop of fodder rape to be sown over the same site last year, to evaluate the longer-term effects of the fertiliser treatments on crop yields.

“The results were very promising. Ripping with fertiliser had a marked effect, while ripping with no fertiliser made no difference at all,” Mr Dean said.

“Using poultry manure led to a 70 per cent increase in dry matter. FeedTest results also showed crude protein content of the manured plots was 84 per cent higher.

“These are very significant boosts given that you can usually expect only 5 to 10 per cent improvements in yield from other measures, like using new varieties or changing crop agronomy.”

Mr Dean said the variations in yield were still being discussed and may be due to the transformation of the soil by root growth and microbes, which improved soil structure and enabled the soil to hold more water.

“In particular, fungi and bacteria that feed on the manure produce sugar proteins such as glomalin, a kind of super-glue that binds particles of clay together to make the soil more like breadcrumbs,” he said.

“As the soil becomes more aggregated, it allows better water infiltration, holds more water for plant use and keeps pore spaces open for air to circulate.”

The Tasmanian site is now part of a national project funded by the GRDC to further explore the mechanisms behind subsoil manuring, how long the effects will last and alternative sources of organic matter.

Along with sites in South Australia, Victoria and New South Wales, a new trial was established in April this year at Nile in Northern Tasmania on the farm of Michael and Fiona Chilvers.

“We’re keen on extending this research into the use of on-farm plant material, such as crop residues, to reduce subsoil manuring costs and the reliance on animal manures,” Mr Chilvers, a member of the GRDC High Rainfall Regional Cropping Solutions Network, said.

Whether the use of subsoil manuring to improve duplex soils becomes more common will depend on costs, which are also being investigated in the project.

“While application costs may be around $1300 per hectare, previous economic analyses have suggested that the resulting improvements in crop yields could pay this investment off in around three years,” Mr Dean said.

For more information about the subsoil manuring project email geoffrey.dean@utas.edu.au.

NRM North and GRDC are supported by funding from the federal government.

The institute is a joint venture between the University of Tasmania and the Tasmanian government.